The investigations carried out in the last years by the Ostia Marina Project (University of Bologna – Dipartimento di Storia Culture Civiltà) in the neighborhood outside Porta Marina at ancient Ostia begin to clarify not only the development of the coastal landscape, but also the dynamic forms of urbanization that unfold from the 1st century B.C. until the 4th century A.D. At the end of the 4th century this suburban area was transformed in one of those ‘sordid places’ cited by a well-known inscription, where prestigious buildings such as the sumptuous building explored by Giovanni Becatti or the Marciana Baths stood side by side with dark workshops and remote meeting places. In this context a caupona, ex-novo designed and built in the middle of the 3rd century, was renovated and adapted for a Mithraic community. The building, called ‘Mithraeum of coloured marbles’, was endowed with a very special worship hall with a marble pavement and walls plastered and painted in imitation of fine marbles. The interpretation of the building as well as the refinement of its chronology are made particularly complex not only because of the uniqueness of the monument, but also by the specific complexity of the stratigraphic sequence, largely conditioned by the activities of robbery implanted in the site after the year 1000.